The present invention relates to contact image sensors for use in document scanning devices and to a method of making contact image sensors.
Known image sensing devices consist of an array of picture elements comprising an array of individual electrodes separated from a common electrode by a photoconductive material. When light falls on the device, current flows between the individual electrode and the common electrode of each illuminated picture element. Detection of the current for each picture element provides an electrical signal pattern which is indicative of the image detected.
FIG. 1 shows one known type of image sensor structure employed as a contact image sensor for facsimile equipment or the like. FIG. 1(a) is a plan view of the image sensor, and FIG. 1(b) is a sectional view of the sensor taken along the line A--A in FIG. 1(a). Transparent individual electrodes 2 each having a contact portion 21 and a lead portion 22 are arranged in a row on a glass substrate 1. This type of image sensor is formed in such a manner that an electrically conductive transparent thin film such as indium-tin oxide (ITO) is formed on the whole surface of the substrate to a thickness of 500.ANG. to 2,000.ANG. by either electron beam evaporation or sputtering, and then shaped in a pattern by photolithography and etching. Then, a metal film is deposited, and metal conducting strips 3 which are in contact with respective lead portions of the transparent electrodes are formed therefrom by photolithography and etching. This metal film may be made of a single metal, such as Cr, Al, Mo, W, Ni, Cu or Au. Alternatively, the metal film may advantageously comprise three different metal layers, e.g., Cr (thickness: 500 to 3,000.ANG.), Ni (thickness: 1,000.ANG. to 1.0 .mu.m) and Cu (thickness: 500 to 3,000.ANG.) to better withstand subsequent processing steps and to improve adhesion of the metal film to the glass substrate.
An amorphous silicon (a-Si) photoconductive layer 4 is formed thereon by glow discharge of silane gas. This a-Si layer 4 is formed so as to cover the contact portions 21 of the transparent individual electrodes by employing a metal mask. Examples of the a-Si layer 4 include one in which an undoped a-Si layer of 0.5 .mu.m thickness and an n-type a-Si layer of about 500.ANG. thickness are laminated to employ an ITO/a-Si heterojunction. In another example, a p-type a-Si layer of about 100.ANG. thickness, an undoped a-Si layer of 0.5 .mu.m thickness and an n-type a-Si layer of about 500.ANG. thickness are laminated to employ a pin junction. In place of the p-type a-Si layer, a p-type a-SiC:H may be employed.
A metallic common electrode 5 is formed on the a-Si layer 4 by either evaporation or sputtering. The common electrode 5 may be made of Al, W, Cr, Ni, etc. and formed in a pattern using a metal mask during the evaporation or sputtering. In the foregoing manner, a row of picture elements are formed, in which light signals are received through the glass substrate 1.
FIGS. 2 and 3 illustrate other known image sensor structures. The illustrated contact image sensor in FIG. 2 is formed as follows. An electrically conductive paste is applied to an insulative substrate 1 by screen printing and subjected to photolithography and etching to form conducting strips 3. After evaporation of a chromium layer, photolithography and etching is carried out to form individual electrodes 2 having contact portions 21 and lead portions 22 in electrical contact with respective conducting strips 3. Then, a photoconductive film 4 is formed using a mask so as to cover the contact portions 21. For example, an intrinsic amorphous silicon hydride film (a-Si) can be formed by plasma CVD employing a mixture of silane gas and hydrogen gas. Then, a transparent common electrode 5 is formed by evaporation of indium-tin-oxide (ITO) using a mask so that the common electrode 5 overlies the contact portions 21 separated by the a-Si film 4.
FIG. 3 shows a third contact image sensor according to the prior art. This image sensor is formed as follows. Chromium and gold are deposited on a glass substrate 1 by evaporation, and photolithography and etching is carried out so that the contact portions 21 and lead portions 22 comprise a single layer of chromium, and the metal conducting strips 3 comprise two layers of chromium and gold. Then, p-type amorphous silicon hydride carbide (hereinafter referred to as "a-SiC"), intrinsic a-Si and n-type a-Si are successively deposited by plasma CVD to provide a photoconductive film 4. Then, ITO is deposited by evaporation to form the common electrode 5, and chromium film is further deposited by evaporation. Photolithography and etching are carried out to provide openings in the chromium layer which are directly above respective contact portions 21 of the individual electrodes 2. Thus, a transparent common electrode 5 having a light-shielding film 7 is formed.
Image sensors such as those shown in FIGS. 1 through 3 have several drawbacks. For example, the image sensors shown in FIGS. 1 and 2 involve variations in the overlap of the common electrode with the individual electrodes (denoted by x in the figure), since the common electrode 5 is formed by evaporation using a mask. At one extreme of the overlap x, the overlap between the common electrode 5 and the lead portions 22 is large, and the effective areas of the picture elements are increased. At the other extreme of the overlap x, the effective areas of the picture elements are reduced because the common electrode 5 does not completely overlap the contact portions 21. Thus, even though the areas of the individual electrodes 2 are constant, variations in the overlap x result in variations in the effective areas for photoelectric conversion of the picture elements. Consequently, variations in the photoelectric output of a picture element, which is proportional to its effective area, of .+-.30% have been observed. Such variations result in sensing devices of lower quality and reduces the production yield. In addition, since the ITO film deposited by evaporation is formed into a common electrode 5 of a desired configuration by photolithography and etching, the etching solution contacts the a-Si film 4. This lowers the quality of the a-Si film, resulting in a further reduction in the production yield.
The image sensor shown in FIG. 3 has a structure which overcomes the foregoing disadvantages. This structure, however, has an undesirable current leakage path between the metal conducting strip 3 and the common electrode 5 across the exposed sidewall A of the photoconductive film 4. Consequently, the leakage current of each picture element is large when there is no protective film covering the sidewall A. Such a leakage current leads to variations in the output of the sensor and a lowering in its reliability in terms of sensitivity to moisture and heat. To overcome this problem, an effective passivation technique in the form of an expensive protective film must be used, resulting in an increase in costs of the image sensor.
Another problem encountered in forming known image sensors arises during deposition of the common electrode onto the semiconductor film. This step involves depositing metal on the semiconductor film by, for example, vacuum evaporation or sputtering. If the semiconductor film has even minute pinhole defects, the metal electrode material may fill the pinhole, leading to a short circuit between the common electrode and an individual electrode. For example, in the case where the semiconductor film is an a-Si film and titanium or chromium is employed as the common electrode, a considerable number of short circuits are produced, resulting in a considerable reduction in the production yield. A granular aluminum film which cannot easily penetrate minute pinholes in the semiconductor film may be grown by evaporation. Therefore the use of aluminum as the common electrode provides a relatively high production yield. However, aluminum has poor resistance to corrosion and is easily diffused into a-Si. Consequently, the use of aluminum as the common electrode is not acceptable from the standpoint of reliability.
It is an object of the present invention to eliminate the above-described disadvantages of the prior art and provide a contact image sensor which has reduced variations in the photoelectric output among light-receiving elements, a reduced leakage current and reduced costs.
It is a further object of the present invention to overcome the above-described problems of the prior art and provide a highly reliable image sensor which is free from short circuits even when the photoconductive semiconductor film has pinhole defects.
It is a further object of the present invention to provide a manufacturing method for image sensors according to the invention, and particularly a method which provides baking at elevated temperatures or formation of an insulating film provided for the purpose of limiting the photoelectric conversion area of the picture element portion, thereby allowing an image sensor having stable characteristics.